Within the data processing industry, there has been much effort directed to increasing the reliability of a computer system and to providing, within each system, a self-diagnosing ability to permit the accurate detection and correction of malfunctions. The increased use of standardized integrated circuits and circuit modules has enabled the construction of an entire computer system on fewer than 50 circuit boards, and common procedure for servicing data processing systems in the field is to identify the circuit board or boards wherein a malfunction occurs and simply swap or replace the identified boards with substitute boards. The replaced boards are not tested and repaired in the field but are sent back to a centralized testing facility for in-depth evaluation and necessary repair.
Although this manner of field service has substantially decreased the amount of time necessary to correct a malfunction in a computer system, it has given rise to a significant hidden cost of repair. This cost resides in the cumulative value of the circuit boards constantly in transit between the field locations and the central testing facility and in the inventory of the circuit boards at the testing facility waiting for diagnosis and repair. It has been found that many of the prior art devices and procedures for diagnosing the sources of a computer malfunctions in the field have provided indefinite or incomplete information with regard to the source of the errors resulting in the replacing of many properly functioning circuit boards in addition to circuit boards that are indeed malfunctioning. These "good" boards are also transferred to the central testing facility and the end result is a significant increase in the cost of repairing computer systems arising from the expense of transferring the properly functioning circuit boards to and from the central testing facility, the wasted expense arising from testing a properly functioning circuit board, and the need to have in service a significantly greater number of additional circuit boards because of the inefficiencies and inaccuracies in the diagnosing and repair operations.
Thus, a significant need has arisen for a quality logic test (QLT) apparatus and procedure for completely testing the individual units or elements in the computer system as well as for testing the interfaces between the units. Such a QLT apparatus should give an operator a quick confidence test of effective, the confidence check should include not only the registers, data paths, etc. in the system elements themselves, i.e. the individual processors, system memory, and I/O controllers, but should also evaluate the functioning of the interfaces between the system elements. Further, the QLT apparatus should function in a modular manner wherein selected portions of the entire computer system are tested in sequence with some means of communication being provided to indicate whether or not a tested portion is malfunctioning. Finally, the QLT apparatus should identify with the highest degree of specificity possible the particular circuit board wherein a diagnosed error occurs.